Summing-up all individual integers the best way?

I just need a little help on how to create this program the precise way. Example if the user will enter 123450, it should output the sum of all individual integers, in the example the result must be 15. If the user will enter a character or a negative number, an error msg will be displayed.

Problem's on line 15. Obviously misunderstood what valueOf does.
You need to use either substring or charAt there to pick out each char of the user's input string, then you can use parseInt to convert that one character to a number.
There are better ways to handle each single character, but maybe it's best to get this working with what you already know, then later we can move on to discuss better ways to do it.

Anyway, charAt returns a char, so that's not a valid parameter for parseInt. YOu can keep the parseInt but use substring instead of charAt, or you can keep the charAt and use the fact that char is a numeric type, so, for example
myChar - '0' returns 0 if myChar is '0', 9 if myChar is '9' etc

Returns a new string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex - 1. Thus the length of the substring is endIndex-beginIndex.

So substring(i,i) will return a string of zero characters length. substring(i, i+1) would be better.

Also line 13 <= intNumbers.length() - that will give you values from 0 to length, which is one value too many.